1. Field of the Invention
This invention relates to radio frequency mixers and particularly to mixers for suppressing harmonic conversion signals used in radio frequency tuners.
2. Prior Art
A mixer performs a combining operation on two input signals to create an output signal. In radio frequency (RF) applications, a local oscillator (LO) input signal mixes with the other input signal to achieve frequency translation, generally called up-conversion or down-conversion. In an up-conversion configuration, the input baseband signal is up converted to an intermediate frequency (IF) or RF signal, or an IF signal is up converted to an RF signal. In a down-conversion configuration, the input RF signal is down converted to an IF signal, or an IF or RF signal is down converted to a baseband signal. A baseband signal can be either a single real valued signal or a complex valued signal comprising an in-phase (I) and quadrature-phase (Q) signal. Conversion to or from a complex baseband signal requires two mixers, one mixer is driven by an in-phase LO and the other mixer is driven by a quadrature-phase LO. In the case of up-conversion, the other mixer ports are driven by the I and Q baseband signals, and the output of the two mixers is summed to create an IF or RF signal. In the case of down-conversion, the IF or RF input is split to drive two mixers, and the output of the two mixers produces the I and Q baseband signals.
A tuner is a combination of circuits used to select and down convert a single channel, or band of channels, from a wideband frequency division multiplex (FDM) channel. Examples of uses for tuners include over-the-air (OTA), satellite, and cable television receivers. A tuner selects a single TV channel, a narrow band, from the broadband RF spectrum, and outputs a band limited signal to a TV or to other circuitry for further processing.
A tuner uses various combinations of mixers, RF low noise amplifiers (LNAs), RF preselect filters, carrier tracking and other forms of frequency control, frequency synthesized LO or LO controlled by an automatic frequency control (AFC) loop, and filters to tune a selected channel.
Common tuner configurations include double conversion, single conversion, and direct conversion. A single conversion tuner, also called a heterodyne, converts the received RF signal to an IF signal using an LO frequency that is the sum or difference between the RF and IF signal frequencies. The IF signal is down converted to a baseband signal outside of the tuner for demodulation or can be demodulated directly from the IF signal. A double conversion tuner converts the incoming RF signal to a first IF signal, followed by a second conversion to a second IF signal or to a baseband signal. The second IF signal is demodulated or down converted to a baseband signal outside of the tuner. A direct conversion tuner, also called a homodyne, converts the RF signal directly to baseband using an LO frequency that is the same as the RF signal frequency.
Prior art tuners are described in the following references, each is incorporated herein by reference: Birleson et al., U.S. Pat. No. 6,177,964, issued Jan. 23, 2001, entitled “Broadband integrated television tuner”, which discloses an up-conversion mixer and a down-conversion mixer in series to convert an RF signal to an IF signal; Rotzoll, U.S. Pat. No. 5,737,035, issued Apr. 7, 1998, entitled “Highly integrated television tuner on a single microcircuit”, which discloses a receiver using an up-conversion mixer and a down-conversion image rejection mixer in series to produce an IF signal.
To avoid degrading the signal that is processed by the tuner, the mixers should have low noise characteristics. Two common types of mixers are used in tuners: multiplying mixers and switching mixers. Multiplying mixers produce an analog output from analog inputs; the two inputs are multiplied linearly to produce the output. Switching mixers are not linear due to discontinuously switching the input signal with the LO to produce the output. Multiplying mixers have higher in-band noise, while switching mixers have lower noise but have strong gain at harmonics of the LO frequency, specifically odd harmonics due to the square wave switching action. The harmonic conversion gain is undesirable in broadband systems such as TV because interfering signals could reside at frequencies that are converted by the LO harmonics to the output frequency of the channel of interest. The harmonic interferers could be other TV channels, resulting in interference into the tuned channel.
Prior art mixers are disclosed in the following references, each incorporated herein by reference: Somayajula, U.S. Pat. No. 6,560,451, issued May 6, 2003, entitled “Square wave analog multiplier”, which discloses square wave analog mixer embodiments for heterodyning operation; Filoramo et al., U.S. Pat. No. 6,433,647, issued Aug. 13, 2002, entitled “Low noise I-Q mixer”, which discloses a low-noise quadrature phase I-Q modulator having a pair of Gilbert cell stages driven by a square wave LO. Atherly et al. U.S. Pat. No. 5,140,198, issued Aug. 18, 1992, entitled “Image canceling mixer circuit on a integrated circuit chip”, which discloses doubly balanced mixers injected with quadrature phase LO, followed by another 90 degree phase shift at the output of one mixer and summing of the two mixer outputs in order to reject one of the mixing terms.
One approach to solving the harmonic conversion problem is the use of harmonic-rejection mixers. This approach has been described in Weldon et al., “A 1.75-GHz highly integrated narrow-band CMOS transmitter with harmonic-rejection mixers”, IEEE Journal of Solid-state circuits, Vol. 36, No. 12, December 2001. This paper described the use of a harmonic-rejection mixer in a transmitter. The harmonic-rejection mixer described uses a 3-bit amplitude quantized sinusoid for the LO. This shifts the harmonics of the LO to 7 times the LO frequency, relaxing the post mixing filter requirements. The sinusoid approximation is formed by using phase-shifted square waves to drive 3 mixers with weighted current drive, and the outputs are current summed to produce the composite mixer output. A separate group of mixers is used for the I and Q components of the signal. This mixer architecture configured for up-conversion in a transmitter and is not suitable for use to down convert an RF or IF signal to a complex baseband signal.